This invention relates to a utility bridge spanning the distance between adjacent facilities subject to relative motion due to wave action on at least one of the facilities and, more particularly, to a utility bridge for connecting offshore platforms which provides a substantially rigid frame disposed between the platforms in such a manner that it will not couple the dynamic response of the respective platforms and a plurality of helical pipelines having torsional expansion loops carried across the frame and establishing a multiplicity of flow lines between the adjacent offshore platforms.
In certain situations it is desired to place a second offshore platform in very close proximity to a first such platform. In some instances the most economic production of oil and gas reserves may justify a second platform nearby an existing platform. In other instances, the efficient production of reserves tapped through drilling from a first platform may be found to be best transported only after preprocessing of the production fluids in on-site facilities requiring a separate production platform installed immediately adjacent the first platform.
In these and other situations requiring a plurality of closely spaced offshore platforms, it is often necessary or desirable to provide for the transport of production fluids from one platform to another and various control and working fluids may also be most economically handled with a common processing facility. Further it is desirable to provide a convenient means for personnel to move between the platforms. However, even conventional rigid leg platforms sway in response to the natural environmental forces in all but the shallowest of offshore waters. This action is accentuated with the height of the structure and in deep water the dynamic response of the platform commonly results in a deflection of several feet at the height of the platform deck.
This dynamic response of offshore platforms creates a problem in joining the adjacent platforms with a bridge because differences in the design, orientation, weight distribution, natural period, time delays between the impact of wave forces to the respective platforms and a myriad of other factors cause the platforms to move independently. A rigid linkage dynamically coupling the tops of the respective platforms would substantially and adversely affect the loading of the platforms by resisting independent movement. Therefore, any walkway or piping joining adjacent offshore platforms must not dynamically couple the platforms' respective responses to natural environmental forces.
Truly flexible means for transporting fluid between the towers generally lack the strength and reliability of pipeline connection. Further, flexible alternatives with higher strength characteristics such as metal woven hose may not be attractive after economic consideration in relation to pipeline. Therefore, a suitable pipeline configuration for fluid transfer would be preferred.
The prior art has provided a bridge between offshore platforms in which a substantially rigid frame having a lost motion sliding joint at one end supports a pipeline having a conventional U-shaped expansion loop. The expansion loop of this known embodiment disperses elastic strain in response to relative platform movement as localized bending moments in a relatively limited portion of the pipeline at the conventional expansion loop. Such repeated and concentrated loading of the pipeline at the expansion loop shortens its useful life. Further, this configuration of expansion loops is not conducive to support and mounting on the bridge itself. Rather, the U-shaped expansion loops require utilization of the very expensively provided platform deck space. Even apart from the relative expense, the U-shaped expansion loop configuration seriously limits the number of pipelines which can communicate between the platforms across a single bridge.
In addition, the relative expense of offshore construction favors modular construction in which a bridge comprising a frame, walkway and complete pipeline assembly is suitable for onshore fabrication and subsequent transportation to the offshore location for installation. This further discourages the use of U-shaped expansion loops which must be fabricated on deck and particularly discourages the use of torsion expansion loops on the platform deck. The later alternative has also been found to be inefficient in the utilization of platform deck space and relatively limiting in the number of pipelines which can be accomodated.